This invention relates to spinal surgery, and more particularly to an anterior cervical fusion compression plate.
The human spinal column is a structure consisting anteriorly of the individual vertebral bodies and the intervertebral discs. The posterior aspect of the spine consists of joints and ligaments which primarily provide a tension band for the spine. The anterior column tends to have characteristic curvatures while viewed from the sagittal plane (from the side) in the cervical, thoracic and lumbar areas, the cervical area having a lordotic or forward bow curve. These various curves are important to be maintained since they allow the spine to have sagittal balance and be agile through normal range of motion when stressed by the various contractions of muscles, etc. Disc degeneration which can lead to disc herniation as well as spur formation usually causes a lessening of the normal cervical lordosis and may lead to significant pain involving the neck as well as radicular pain due to nerve root compression. Tumors, disease, trauma and degeneration are all factors which can lead to spinal pathologies requiring fusion.
The usual approach to diseases of the spine, when they do not respond to conservative therapy, includes operation. Operations have approached the cervical spine with either an anterior approach, involving the vertebral bodies and discs, or a posterior approach, which allows for decompression of the nerve roots as they exit the spinal foramina and a very limited degree of discectomy via laminotomy. For derangements of spinal alignment, large herniated discs, compression of the spinal cord due to discs or spurs or fracture, the anterior approach is generally favored. The anterior approach also allows for correction of spinal deformity. When the intervertebral disc is removed or offending bone fragments are removed, the cavity which is created in this surgery is generally filled by a graft material. Graft materials have been procured from the patient themselves (autograft) as well as from bone bank bone (allograft) and then in certain circumstances synthetic materials are used, such as PEAK, corral and synthetic porous materials.
A number of factors have been identified as being necessary to achieve a solid bony fusion across an operated site. These factors include: 1) relative immobility across the operative site, to limit movement of the graft material relative to the surrounding metabolically active bones, 2) compressive loading force across the graft site, 3) healthy metabolically active bone, and 4) adequate blood supply. Regarding the second factor, compressive loading force across the graft site, this compressive force allows the avoidance of gapping between the graft material and the metabolically active bone which is attempting fusion with the graft material. The mechanism of bone formation with grafting, termed “creeping substitution” causes to a small degree loss of graft material, thus possibly forming a gap. This is due to a resorptive phase in the bone formation mechanism. This bone resorption can result in gaps between the bone graft and the metabolically active bone and if the juxtaposition of graft and surrounding bone is not close enough, or if there cannot be a mechanism by which the level can compress together and maintain that close contact, a pseudoarthrosis or false fusion can occur.
Studies have been performed to evaluate cervical fusions, and specifically studies have looked at cervical discectomy with fusion when performed at one, two and three levels. It has been found that the fusion rate at a one level in the spine is much higher than at a second level if done at the same sitting, and a third if done at the same sitting is lower than the second and first levels. This is felt to be due to an inability to maintain adequate stabilization of the spine during postoperative healing at the second and third levels as compared to the first level.
In an effort to enhance stability in post surgical cervical fusion patients, anterior cervical plating has been utilized. In these constructs, the anterior cervical plating system is rigidly affixed to the bones above and below each level fused in an effort to maintain stability. The compression across the graft is provided by slightly over sizing the graft within the gap created by removal of disc and the resultant compression is provided by the counteractive force of the stretched spinal ligaments.
When considering a rigid anterior cervical plating system, a certain amount of compressive force can be placed across the graft during the initial placement of the plate by angling the screws above and below the level of the graft, and lagging the plate to the vertebral body. In this instance, the stored compression may not be enough to allow for the process of creeping substitution described earlier. The plate then will begin to act as a fixed cantilever and actually may allow the surrounding bone to be held apart from the graft material. The process of creeping substitution may then allow gaps to form thereby leading to pseudoarthrosis. This ultimately can lead to pain and/or deformity and require further surgeries. This is termed a distraction pseudoarthrosis.
Other cervical plating systems have been introduced in an attempt to prevent this distraction pseudoarthrosis by allowing the vertebral bodies to move towards each other as needed during the fusion process. This has generally been done by allowing bone screws to be free to rotate, swivel, slide or angulate independent of the anterior cervical plate itself. These screws are free to carry out these actions despite the fact that they are fixed from backing out of the plate. This does allow some freedom of motion of the vertebrae to move toward each other during the fusion process but does lead to a degree of multi-directional instability that is contrary to the purpose of the anterior cervical plating hardware, which is to provide increased stability.
Yet another approach to addressing the issue of providing compressive force across the graft has involved a plating system by which a bar is attached to each vertebral body and each of the bars can slide on a pair of rods. These constructs do allow movement of the bone across the graft surface, however they do sacrifice stability in the sense that the vertebral bodies attached to the sliding bars can easily slide away from the graft as well, particularly when the patient in the supine position when the head is not weighting the graft. These types of systems sacrifice stability in the sagittal plane to allow for the compressive force generated by the head when the patient is in the upright position. The second issue is the compressive force that is provided across the graft is provided solely by the weight of the patient's head and therefore these systems are entirely ineffective if the patient is in the supine position, and in fact, if the patient is in the supine position and the head is levered around an object such as a cervical collar, it can actually be a distractive force across the fusion site which is definitely contrary to the purpose of the surgery.
U.S. patent application Publication No. US 2002/0188296 teaches a plate that allows not only for the sliding of the plate from an elongated to a collapsed status, a process termed by the inventor as dynamization, but the plate also does not allow for challenged distraction. This plate would therefore allow a compressive load to be applied to the graft within the physiologic range (weight of the head), and if the graft dissolves to a degree, the plate allows the vertebral bodies to move towards each other, but not away. This prior art plate also teaches what the inventor calls “active dynamization” by which the plating system is forced to store energy to induce shortening of the fusion construct should the opportunity present, primarily due to the maneuvers discussed herein before, including over sizing the graft and diverging the screws. This concept may be considered as “dynamization”, but the designation of this concept as being “active” dynamization would, in the view point of the present inventor, be inaccurate as there is no active element for applying compressive loading.
The limitations of this prior art plating system are the same as with other subsidence systems in that the compressive load across the graft is supplied by the patient's head and is only present at the time when the patient is upright. This force is very readily removed as soon as the patient is in the supine position. This prior art plate however does not have the negative that is shared by other systems of the prior art such as found in U.S. Pat. No. 6,402,756, as well as in U.S. Pat. Nos. 5,616,142, 5,800,433 and U.S. Publication No. 2002-0111630, which have distraction across the graft site in the supine position. U.S. Pat. No. 6,328,738 discloses another prior art plate which, similar to the other prior art mechanisms, would prevent distraction across the graft site, but is ineffective in providing any active compression.